Apparatus and method for testing conductivity of graphene

ABSTRACT

According to the present invention, oxidized and reduced regions of graphene can be accurately detected in a short time using a terahertz wave so as to measure the conductivity of graphene, and thus the time required to test the conductivity of graphene can be reduced. In addition, when an oxidized region exists in graphene, the oxidized region can be immediately reduced by irradiating an electromagnetic wave thereto so as to increase the conductivity of graphene and thus minimize the time required to restore graphene.

TECHNICAL FIELD

The present invention relates to an apparatus and method for testing theconductivity of graphene, and more specifically to an apparatus andmethod for testing the conductivity of graphene through the detection ofoxidized or reduced regions of the graphene.

BACKGROUND ART

Graphene is a 2-dimensional structure of carbon atoms arranged in ahexagonal crystal lattice. Graphene has high electrical conductivity,high thermal conductivity, and high mechanical stiffness despite itssmall thickness (˜3.4 Å). Due to such characteristics, graphene has beenin the spotlight as a material for semiconductor devices that has thepotential to replace silicon in the near future. The high electricalconductivity and high mechanical stiffness of graphene make it easier toproduce flexible substrates. Based on these physical properties,graphene has attracted attention as a transparent electrode materialcapable of replacing indium tin oxide (ITO).

Graphene oxide is stable due to its high solubility. Accordingly,graphene is stored and transported in its oxide form and graphene oxideis reduced for use where conductive graphene is required. However,graphene oxide is not completely reduced. In some cases, reducedgraphene oxide is again oxidized. Test methods for graphene are thusconsidered important.

According to conventional methods for testing mass-produced large-areagraphene, the presence or absence of defects in graphene is determinedby observing a change in temperature distribution after a current isapplied to the graphene. Large-area graphene loses its conductivity whenpartially oxidized. In this case, the application of current causes adifference in electrical resistance between oxidized and reduced regionsof the graphene. The different resistance values lead to a difference inthe amount of heat generation upon the application of current, and as aresult, the thermal distribution of defective regions (oxidized regions)is distinguished from that of defect-free regions (reduced regions). Byinspecting the different thermal distributions using a thermal imagingcamera, a determination can be made as to whether the graphene isdefective or not.

However, when the defective regions are monitored through their thermaldistributions, it is impossible to determine the exact position and sizeof the defective regions. No report has appeared on more preciseapparatuses and methods for testing mass-produced graphene to determinethe position and size of defective regions in the graphene.

In this connection, Korean Patent Publication 10-2013-0114617 disclosesa method for testing graphene substrates using ultraviolet light.However, this method requires a darkroom to use ultraviolet light,causing inconvenience for users, and utilizes the ultraviolet lighttransmittance of graphene through a difference in the formation ofgraphene layers rather than the electrical conductivity of grapheneitself, limiting its accuracy.

DETAILED DESCRIPTION OF THE INVENTION Problems to be Solved by theInvention

Therefore, an object of the present invention is to provide an apparatusand method for testing the conductivity of graphene in which terahertzwaves are used to measure the conductivity of graphene.

Means for Solving the Problems

One aspect of the present invention provides an apparatus for testingthe conductivity of graphene, including a light processing unit forirradiating terahertz waves onto graphene and receiving the terahertzwaves reflected from or transmitted through the graphene, adetermination unit for detecting the terahertz waves from the lightprocessing unit to detect oxidized and reduced regions of the graphene,and a display unit for imaging data processed in the determination unit.

The terahertz waves irradiated from the light processing unit may betransmitted vertically through the graphene.

A light source for the terahertz waves may be of pulsed or continuoustype and may be provided in plurality, and the terahertz waves may havewavelengths of 30 μm to 3 mm.

The light processing unit includes a holder adapted to fix the graphene,a light emitter placed above the graphene holder and including a lightsource adapted to irradiate terahertz waves, and a photosensor placedbelow the graphene holder to receive the terahertz waves transmittedthrough the graphene.

The apparatus of the present invention may further include a restorationunit for irradiating electromagnetic waves onto the oxidized regions ofthe graphene detected in the determination unit to reduce the oxidizedregions.

The electromagnetic waves may include all wavelengths in theultraviolet, visible, and infrared regions. Specifically, theelectromagnetic waves may have wavelengths of 160 nm to 2.5 μm.

Another aspect of the present invention provides a method for testingthe conductivity of graphene, including (a) fixing graphene to aspecimen stage, (b) irradiating terahertz waves onto the graphene, (c)detecting the transmittance of the graphene for the terahertz waves, (d)analyzing the transmittance for the detected terahertz waves to obtainan image, and (e) detecting oxidized regions of the graphene through theimage.

A light source for the terahertz waves may be of pulsed or continuoustype and may be provided in plurality.

The terahertz waves may have wavelengths of 30 μm to 3 mm.

The apparatus of the present invention may further include irradiatingelectromagnetic waves onto the oxidized regions of the graphene detectedin step (e) to reduce the oxidized regions.

The electromagnetic waves may include all wavelengths in theultraviolet, visible, and infrared regions. Specifically, theelectromagnetic waves may have wavelengths of 160 nm to 2.5 μm.

The test target graphene may be an electrode device or a transparentelectrode.

Effects of the Invention

According to the present invention, terahertz waves are irradiated ontolarge-area graphene to detect the transmittance of the graphene. Thisallows for rapid measurement of oxidized and reduced regions of thegraphene, enabling the detection of the electrical conductivity of thegraphene. In addition, the oxidized regions of the graphene are reducedimmediately after detection. This can shorten the time required torestore the oxidized regions of the graphene, leading to a reduction inthe overall testing time and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the constitution of an apparatus for testing theconductivity of graphene according to one embodiment of the presentinvention.

FIG. 2 is a block diagram illustrating the configuration of a lightprocessing unit of the apparatus illustrated in FIG. 1.

FIG. 3 is a flowchart illustrating the steps of a method for testing theconductivity of graphene according to one embodiment of the presentinvention.

FIG. 4 shows (a) an image of graphene as a detection target and (b) animage of the graphene after testing in accordance with a method of thepresent invention.

FIG. 5 graphically shows reflectance values from some regions of thegraphene shown in (b) of FIG. 4.

FIG. 6 is a partial block diagram illustrating the configuration of thelight processing unit illustrated in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail withreference to the accompanying drawings. It will be obvious to thoseskilled in the art that these drawings are provided for illustrativepurposes only and the scope of the invention is not limited thereto.

FIG. 1 illustrates the constitution of an apparatus for testing theconductivity of graphene according to one embodiment of the presentinvention. The apparatus of the present invention includes a lightprocessing unit 100, a determination unit 200, and a display unit 300.The apparatus of the present invention is characterized by the use ofterahertz waves for measuring the conductivity of graphene.

The light processing unit 100 irradiates terahertz waves onto graphene,receives the terahertz waves reflected from or transmitted through thegraphene, converts the received terahertz waves into electrical signals,and outputs the electrical signals.

The terahertz waves irradiated onto graphene are electromagnetic waveshaving wavelengths of 30 μm to 3 mm in the frequency range of 0.1 to 10THz. Terahertz waves have a strong ability to pass through graphenebecause they are longer in wavelength than visible light and infraredlight. Terahertz waves are available even where external light ispresent, unlike other light waves. Therefore, the use of terahertz wavescan eliminate the need for the step of blocking external light.

A light source for the terahertz waves may be of pulsed or continuoustype. A pulsed light source is more preferred due to its hightransmission through graphene.

The light source for the terahertz waves is included in a light emitter101. The light source for the terahertz waves may be provided inplurality. The use of the plural light sources for the terahertz wavesenables 2-dimensional testing of graphene, leading to a significantreduction in the time required to test the graphene.

FIG. 2 is a block diagram illustrating the configuration of the lightprocessing unit. As illustrated in FIG. 2, the light processing unit 100includes a photosensor 102 and a holder 103 in addition to the lightemitter 101.

The light emitter 101 is placed above the holder 103. After graphene isfixed to the holder 103, the light emitter 101 irradiates terahertzwaves onto the fixed graphene. The terahertz waves irradiated from thelight emitter 101 are transmitted through reduced regions of thegraphene but are absorbed by or reflected from oxidized regions of thegraphene.

Some of the terahertz waves irradiated from the light emitter 101 andincident on the graphene are reflected from the graphene and the otherterahertz waves are transmitted through the graphene. The photosensor102 receives the reflected or transmitted terahertz waves, converts thereflected or transmitted terahertz waves into electrical signals, andtransmits the electrical signals to the determination unit 200. FIG. 6is a partial block diagram illustrating the configuration of the lightprocessing unit. As illustrated in FIG. 6, the photosensor 102 is placedabove the holder 103 to detect the terahertz waves reflected from thegraphene (a of FIG. 6). Alternatively, the photosensor 102 is placedbelow the holder 103 to detect the terahertz waves transmitted throughthe graphene (b of FIG. 6).

The holder 103 may include a chamber 104 and a specimen stage 105. Thechamber 104 may be constructed such that the influence of externalenvironmental factors is minimized to increase the test accuracy. Thespecimen stage 105 may be positioned in the chamber 104. The chamber 104may include an inlet and an outlet through which the specimen stage 105can enter and exit, respectively. However, the construction of thechamber 104 is not limited. The chamber 104 is a part of the holder 103but may include the light emitter 101 and the photosensor 102. Thespecimen stage 105 may be positioned in the chamber 104 to fix thegraphene. The specimen stage 105 is arranged between and in a straightline with the light emitter 101 and the photosensor 102. With thisarrangement, the terahertz waves irradiated from the light emitter 101are vertically incident on the graphene and the terahertz wavestransmitted through graphene can be vertically received by thephotosensor 102.

The specimen stage 105 may be constructed in a roll-to-roll or conveyormanner such that graphene is easily transferred.

The determination unit 200 receives the output signals from the lightprocessing unit 100 and analyzes the reflectance or transmittance of thegraphene for the terahertz waves to detect oxidized or reduced regionsof the graphene. Graphene may be partially oxidized during production,storage or transport. Graphene oxide is unsuitable for use intransparent electrodes, etc. due to its low conductivity and is thusgenerally regarded as being defective. Since the irradiated terahertzwaves are transmitted through the reduced regions but are absorbed by orreflected from the oxidized regions, the degree of reflection ortransmission of the terahertz waves from or through the graphene variesdepending on the reduced and oxidized regions of the graphene. Thereflection or transmission of the terahertz waves allows for detectionof the oxidized or reduced regions of the graphene, enabling themeasurement of the graphene conductivity.

The determination unit 200 may include a detector 201 adapted to detectthe transmittance of the graphene for the terahertz waves and ananalyzer 202 adapted to analyze the detected transmittance. Thedetermination unit 200 may include a storage (not shown) adapted tostore data processed in the analyzer 202.

The display unit 300 displays the data analyzed in the determinationunit 200 on a screen. The distribution of the oxidized regions of thegraphene can be detected through the display unit 300. As illustrated inFIG. 1, the oxidized and reduced regions of the graphene are representedin red and green in the display unit 300, respectively, so that thedistributions of the oxidized and reduced regions of the graphene can beidentified without the need for further processing.

(b) of FIG. 4 is an image of the graphene obtained after irradiation ofthe graphene with terahertz waves. The reduced and oxidized regions ofthe graphene are represented in red and black or blue, respectively, sothat a determination can be made as to whether and where the graphene isoxidized or reduced.

FIG. 5 graphically shows the terahertz waves reflected from positions 1,3, and 12 as a function of time. Most of the terahertz waves irradiatedonto the reduced regions of the graphene are transmitted through thegraphene, which can be explained by the high conductivity of the reducedregions. Meanwhile, most of the terahertz waves irradiated onto theoxidized regions of the graphene are absorbed by or reflected from thegraphene surface in the regions, which can be explained by the lowconductivity of the oxidized regions. The terahertz waves reflected fromposition 1 shown in (b) of FIG. 4 are analyzed and the results are shownin (a) of FIG. 5. The high intensity of the peak indicates thereflection of the terahertz waves from the graphene, demonstrating thatposition 1 is the oxidized region of the graphene. The terahertz wavesreflected from position 3 shown in (b) of FIG. 4 are analyzed and theresults are shown in (b) of FIG. 5. The low intensities of the peaksindicate that the region of position 3 is different from that ofposition 1. That is, position 3 is a pore between the reduced regions ofthe graphene. (b) of FIG. 5 shows the reflection of the terahertz wavesfrom a slide glass as a substrate where the graphene is fixed. Severalpeaks with low intensities appear in (c) of FIG. 5. One of the peakscorresponds to the reflection of the terahertz waves from the slideglass and the other peaks correspond to the reflection of some of theterahertz waves transmitted through the graphene. The transmission ofthe terahertz waves indicates high conductivity of the graphene. Thatis, position 12 is the reduced region of the graphene. In conclusion,the reduced and oxidized regions of the graphene can be detected fromthe results in FIG. 5.

The apparatus of the present invention may further include a restorationunit 400. Whenever the determination unit 200 detects oxidized regionsof the graphene, the restoration unit 400 irradiates electromagneticwaves onto the oxidized regions of the graphene in real time to reducethe oxidized regions. That is, the restoration unit 400 serves torestore the oxidized regions of the graphene. This real-time reductioncan shorten the time it takes to restore oxidized regions of graphenewhen compared to the reduction of detected oxidized regions of graphenein a separate process after testing of the conductivity of the graphene.

The electromagnetic waves include all wavelengths in the ultraviolet,visible, and infrared regions. Specifically, the electromagnetic wavesare waves of white light having wavelengths of 160 nm to 2.5 μm that maybe irradiated by suitable lamps, such as xenon flash lamps and UV lamps.The electromagnetic waves may have a pulse width of 0.1 to 100 ms, apulse gap of 0.1 to 100 ms, and a pulse number of 1 to 1,000.

The present invention also provides a method for testing theconductivity of graphene, including (a) fixing graphene to a specimenstage, (b) irradiating terahertz waves onto the graphene, (c) detectingthe transmittance of the graphene for the terahertz waves, (d) analyzingthe transmittance for the detected terahertz waves to obtain an image,and (e) detecting oxidized regions of the graphene through the image.

FIG. 3 is a flowchart illustrating the steps of a method for testing theconductivity of graphene according to one embodiment of the presentinvention. As illustrated in FIG. 3, in step (a), graphene as a testtarget is fixed to (or loaded on) a specimen stage. The specimen stagemay be positioned in a chamber.

In step (b), terahertz waves are irradiated onto the fixed graphene. Theterahertz waves are vertically irradiated onto the graphene. Theterahertz waves may have wavelengths of 30 μm to 3 mm. The terahertzwaves are highly rectilinear and are thus available even where externallight is present.

In step (c), the reflectance of the terahertz waves from the graphene orthe transmittance of the graphene for the terahertz waves is detected.The detection of the terahertz waves reflected from or transmittedthrough the graphene in oxidized regions of the graphene is differentfrom that in reduced regions of the graphene. For example, grapheneoxide has a low transmittance for the terahertz waves because theterahertz waves are not transmitted through graphene oxide but areabsorbed by or reflected from graphene oxide.

In step (d), the detected reflectance or transmittance is analyzed andimaged. Imaging is performed by plotting the analyzed reflectance ortransmittance to obtain a curve. Alternatively, oxidized and reducedregions of the graphene may be projected onto the graphene to obtain acolor image. As illustrated in FIG. 1, the oxidized and reduced regionsof the graphene are represented in red and green in the display unit300, respectively, so that they can be visually detected through theimage without the need for additional processing.

In step (e), the image obtained in step (d) is used to determine whetherand where the graphene is oxidized. The portions with low transmittancein the image correspond to oxidized regions of the graphene and areexpected to have low conductivity because graphene oxide has lowelectrical conductivity and low transmittance for terahertz waves. Thedata analyzed in step (d) can be compared with the existing data todetect the oxidized and reduced regions of the graphene. The existingdata mean, for example, the reflectance of terahertz waves from graphenewhose conductivity is already known or the transmittance of graphene forterahertz waves. Regions where the detected transmittance of thegraphene for the terahertz waves is lower than the existing data can bedetermined as oxidized regions of the graphene.

The method of the present invention may further include irradiatingelectromagnetic waves onto the detected oxidized regions of the grapheneto reduce the oxidized regions. In this additional step, the oxidizedregions of the graphene are restored. No additional material and noadditional processing, such as annealing, are required, contributing toa reduction in restoration time and testing cost. The electromagneticwaves include all wavelengths in the ultraviolet, visible, and infraredregions. Specifically, the electromagnetic waves are waves of whitelight having wavelengths of 160 nm to 2.5 μm that may be irradiated bysuitable lamps, such as xenon flash lamps and UV lamps. Theelectromagnetic waves may have a pulse width of 0.1 to 100 ms, a pulsegap of 0.1 to 100 ms, and a pulse number of 1 to 1,000.

<Explanation of Reference Numerals> 100 Light processing unit 101 Lightemitter 102 Photosensor 103 Holder 104 Chamber 105 Specimen 200Determination unit 201 Detector stage 300 Display unit 202 Analyzer 400Restoration unit

1. An apparatus for testing the conductivity of graphene, comprising alight processing unit for irradiating terahertz waves onto graphene andreceiving the terahertz waves reflected from or transmitted through thegraphene, a determination unit for detecting the terahertz waves fromthe light processing unit to detect oxidized and reduced regions of thegraphene, and a display unit for imaging data processed in thedetermination unit.
 2. The apparatus according to claim 1, wherein theterahertz waves are irradiated from a pulsed or continuous light source.3. The apparatus according to claim 2, wherein the light source isprovided in plurality.
 4. The apparatus according to claim 2, whereinthe terahertz waves have wavelengths of 30 μm to 3 mm.
 5. The apparatusaccording to claim 1, wherein the light processing unit comprises aholder adapted to fix the graphene, a light emitter placed above thegraphene holder and comprising a light source adapted to irradiateterahertz waves, and a photosensor adapted to receive the terahertzwaves reflected from or transmitted through the graphene.
 6. Theapparatus according to claim 1, further comprising a restoration unitfor irradiating electromagnetic waves onto the oxidized regions of thegraphene detected in the determination unit to reduce the oxidizedregions.
 7. The apparatus according to claim 6, wherein theelectromagnetic waves are pulsed or continuous and have wavelengths of160 nm to 2.5 μm.
 8. The apparatus according to claim 6, wherein theelectromagnetic waves are pulsed and have a pulse width of 0.1 to 10 ms,a pulse gap of 0.1 to 100 ms, and a pulse number of 1 to 1,000.
 9. Amethod for testing the conductivity of graphene, comprising (a) fixinggraphene to a specimen stage, (b) irradiating terahertz waves onto thegraphene, (c) detecting the terahertz waves reflected from ortransmitted through the graphene, (d) analyzing the detected terahertzwaves to obtain an image, and (e) detecting oxidized regions of thegraphene through the image.
 10. The method according to claim 9, whereinthe terahertz waves are irradiated from a pulsed or continuous lightsource.
 11. The method according to claim 9, wherein the terahertz wavesare irradiated from one or more light sources.
 12. The method accordingto claim 9, wherein the terahertz waves have wavelengths of 30 μm to 3mm.
 13. The method according to claim 9, further comprising irradiatingelectromagnetic waves onto the oxidized regions of the graphene detectedin step (e) to reduce the oxidized regions.
 14. The method according toclaim 13, wherein the electromagnetic waves are pulsed or continuous andhave wavelengths of 160 nm to 2.5 μm.
 15. The method according to claim13, wherein the electromagnetic waves are pulsed and have a pulse widthof 0.1 to 10 ms, a pulse gap of 0.1 to 100 ms, and a pulse number of 1to 1,000.
 16. The method according to claim 9, wherein the graphene isan electrode device or a transparent electrode.